1. Field of the Invention
The present invention relates to an optical system controlling apparatus suitable for use in a video camera or the like and, more particularly, to a drive controlling apparatus for an element, such as a lens, which varies optical characteristics.
2. Description of the Related Art
The recent development of video equipment such as a video camera is remarkable and a majority of lens systems utilize autofocus (AF) which automatically detects an in-focus state. In this kind of AF system, it is necessary to detect the amount by which a focus adjusting lens is made to move forwardly in practical use. To meet this requirement, the AF system is provided with detecting means, for example, an encoder such as a slide type of variable resistor or a photosensor. A lens system whose focus adjusting lens is driven by a pulse motor or the like utilizes detecting means which is arranged to detect the required amount of forward movement of the focusing lens by counting the number of drive pulses.
In general, in a system called an inner focus type lens system or a rear focus type lens system in which focusing is performed by driving a rear lens group of the lens system, an arrangement utilizing the aforesaid pulse motor is used because no large drive torque is needed and because compensation for the movement of a focal plane during zooming can be easily performed.
FIG. 10 is a schematic view showing one example of the lens construction of this kind of inner focus type of lens system. The example shown in FIG. 10 includes a fixed first lens group 101, a second lens group (zooming lens) 102 for effecting variation of magnification, an iris 103, a fixed third lens group 104, a fourth lens group (focusing lens) 105 for performing a focus adjustment function as well as the function of compensating for the movement of a focal plane resulting from the magnification varying operation of the second lens group 102, and an image sensing plane 701.
In this kind of lens system, the same lens 105 is used to realize both compensation for the variation of the position of the focal plane during zooming and focus adjustment for focusing a subject. Since high accuracy is required for position control of the lens 105, it is necessary to accurately and rapidly detect the position of the focusing lens 105.
In a specific arrangement, a leaf switch or the like is provided for detecting the initial set position which has been set near one end of the movement range of the focusing lens 105, and the position thereof is accurately detected by opening and closing the switch in response to the movement of the focusing lens 105. For example, when a power source is turned on, the focusing lens 105 is made to move in the direction of the switch and the position of the focusing lens 105 is reset to a reference position.
The initial reset operation of the focusing lens which is performed in this kind of lens system when the power source is turned on, i.e., a sequence of reset operations for moving the focusing lens to open and close the switch for detection of the initial set position, is in essence unfavorable for an automatic focus adjustment operation itself. Accordingly, immediately after the power source is turned on, it is necessary to complete that sequence of operations in a minimum time.
In general, a mechanically operated switch produces chattering at the instant of the opening and closing operation thereof. If the opening and closing operation of the switch is not detected after the chattering is eliminated in a control circuit for a lens driving motor, a variation occurs in the detection timing of the reference position of the focusing lens, with the result that an error occurs in the position of the focusing lens.
To eliminate the chattering of the mechanical switch, it may be considered that reading of the opening and closing operation of the switch is continued during the occurrence of the chattering. However, since a certain length of time is taken until the chattering settles, it is necessary to drive the pulse motor to move the lens during that time as well. As a result, an error due to a time difference occurs between the position occupied by the lens when the switch is actually opened and the position occupied by the lens when the motor controlling circuit detects the opening and closing operation of the switch and resets the count of the pulse motor to a predetermined value.
However, if the driving speed of the pulse motor is slowed to minimize the time difference, it becomes impossible to perform a rapid reset operation.
In a case where a motor speed during the reset operation is constant, if the time required for the whole reset operation is reduced, a detection error as to the reset position will increase. If the detection accuracy of the reset position is improved, there will occur the contradictory problem that the whole reset operation requires a long time.